The invention relates to a micromechanical device, such as an acceleration sensor, angular rate sensor, inclination sensor, or angular acceleration sensor, in which a seismic mass is used as sensing element.
Many devices having micromechanical structures are currently known. One problem associated with many of such structures is that manufacture of the devices introduces internal stresses in the structure and participating in the components which measure the parameter to which the sensor is sensitive. Generation of such stresses causes problems. It often results in the sensor having an offset or varying unpredictably with temperature or over the sensing range of the sensor. This results in each sensor requiring individual testing and appropriate means, either via mechanical or electrical compensation, to be provided in order for the sensor to operate accurately and consistently. It will be appreciated that this can cause a considerable increase in sensor cost, as well as reducing reliability.
Many attempts have been made to overcome the problem associated with induced stress. Most of the approaches are, however, dependent upon employing very specific materials, either in the device components or in the encasing packaging of the device, meaning that are inflexible and cannot be broadly applied to different device types. Many have an additional problem they in requiring extremely complex and costly manufacturing steps which again increase cost and which can be time consuming and result in many rejected devices.
According to the present invention there is provided A micromechanical device comprising:
a pedestal member connected, in use, to a support wall and bonded, in use, to an encasing member; and
wherein the pedestal member has a rim formed around at least a portion of its outer periphery, the rim extending away from the encasing member and supporting at least one sensing component of the device; and
wherein the pedestal member is elongate, with its longer dimension extending in a direction substantially perpendicular to that of the support wall to which it connects.
The device may comprise the support wall which may be arranged such that it surrounds both the pedestal member and the component.
The pedestal member may be bonded to the encasing member in a discontinuous manner.
The component may be connected to the pedestal member by one or more planar flexible hinges.
The micromechanical device may be an acceleration sensor, an angular rate sensor, an inclination sensor, or an angular acceleration sensor.
A gap between the component and the encasing member may be provided and may be formed by an etched recess in the encasing member.
Electrical contacts may be provided with the component or suspension member by the provision of direct electrical contacts located on the edge of the pedestal and on the contact surface of the encasing member. Alternatively, electrical conductors may be provided by implantation of impurities or by sputter deposition of film onto the pedestal structure.
Electrical crossings may be provided perpendicular to the direction of elongation of the pedestal member in order to further reduce stresses in the overall device structure. The device may be formed from silicon.
A method of manufacturing the device is also provided. Within this invention, at least one silicon seismic mass may be joined to a silicon support wall frame via the pedestal structure, the surface of which is bonded to the encasing member which is, either glass or silicon.
This pedestal structure and its method of assembly according to the invention has the advantage that the coupling between the sensing element of the sensor and the frame of the sensor is minimised by using the pedestal member, the bearing surfaces of which are small compared to the surface area from which the device is formed. This reduces the assembly-related strains, stress and associated temperature-induced variations of the overall device, thus simplifying the evaluation electronics of the device.
Other provisions of the structure and its method of assembly according to the invention are also advantageous.
The pedestal member can be fabricated easily, being produced in the same process that structures the micromechanical components of the device, such as a sensing seismic elements and their suspension systems. This structuring process is especially advantageous because well known and established micromechanical structuring processes, such as wet and dry anisotropic silicon etching, can be used for this purpose.
A particular advantage of the pedestal member and its assembly method according to the invention is that the geometry and the manner of its structuring can be selected in accordance with the function of the sensing element and its fabrication sequence.
A special advantage of this invention is that any bonding between the covering wafers and the micromachined silicon wafer, which caries the component, the pedestal member and the support wall, takes place at the wafer level, resulting in economical, easy to handle batch processing. According to the invention, a multitude of ready-structured devices, which have not yet been cut in individual devices, can be bonded simultaneously to the encasing member, then separated, for instance by sawing.
The bonding technique that forms any sealed cavity and anchors the pedestal member to the encasing member is to be chosen depending on the material of the encasing member. If glass is used for covering, then an anodic bonding technique is suitable; if silicon is used, then silicon-to-silicon bonding techniques are advantageously suited. For other materials, soldering bonding techniques can be successfully employed. The atmosphere composition and its pressure can be freely selected and preserved within any sealed cavity by the anodic bonding technique, which makes this technique particularly attractive.
The device and its method of assembly according to the invention allows the optional implementation in its structure of (i) press-contacts, a method of passing electrical conductive paths between the wafers; (ii) buried crossings, a method of passing electrical conductive paths through the bulk of pedestal; (iii) direct crossings, a method of passing electrical conductive paths across the pedestal; and (iv) surface conductors along the pedestal.
The invention particularly enables the realisation of a compact sensor as no other stress-releasing structures or mounting techniques, internal or external, being required.